The present invention relates to magnetic recording and more particularly to a method and apparatus for stabilizing a recorded signal such that other signals can be recorded and written without degrading the stabilized signal.
Magnetic recording, particularly digital signal recording, has been becoming increasingly complicated. Continuing advances have been made in achieving greater linear densities, as well as higher track densities. A recording device designer is now being faced with ever increasing important problems for achieving and reliably building high performance recorders, particularly of the digital type. For example, as track density increases, track seeking and following become increasingly more difficult. Similarly, as linear densities increase, inter-symbol interference becomes more of an acute problem. All of the above problems reach an apex of difficulty when flexible media is used as a record member.
Flexible media in particular require a relatively thick magnetic coating for containing the signal to be recorded. This requirement increases inter-symbol interference problems, particularly at higher linear and track densities. Equalization techniques are used widely for accommodating these problems.
Location of track or track identification techniques at higher track densities also becomes a serious problem. Sector servos, that is, sector or position indicating signals, are interleaved with data signals to indicate the precise location of the data tracks. Sector servo signals reduce the data storing capacity of a record member and therefore becomes expensive. To circumvent the loss of data storing area on a record member, a so-called "buried-servo" has been desired in that the data and the servo then can underlie the same surface area; the servo or position indicating signals ideally occupy a lower portion of the magnetic coating while data signals occupy an upper portion of the magnetic coating directly over the servo signals. A serious problem arises in that when the data signals are repeatedly recorded and reproduced, there has been degradation of the so-called buried servo signal; that is, as more and more data is recorded and retrieved, the signal amplitude of the servo signal gets smaller; this variation makes the system less reliable. In fact, this change occurs at a relatively rapid rate.
Also, in handling high density recording overwriting without erasing is desired. Overwriting in digital records results in certain distortions which can be at least partially solved by the addition of AC bias. However, means are still needed for ensuring that the "buried signal" does not decrease with usage; i.e., it needs to be stabilized.
Further, such "buried" signal can be used for controlling the timing of the recorder for enhancing operation at extremely high linear densities. Unfortunately, to date, no system has been devised for carrying out stabilization of such a signal without having two different coercivity layers of magnetic material. The use of two coercivity layers of magnetic material in a record member greatly increases the cost of the record member. Accordingly, it is desired to provide a stabilized signal in a substantially homogeneous magnetic layer.
Further, in magnetic tape art, an erase gap often precedes a read/write gap. Generally the erase gap has a greater length than the subsequent read/write gap. This gap relationship ensures that the erasing is effective to clear the tape for preventing intermodulation and interference effects in the magnetic layer. However, in reading or sensing signals, a long gap length causes increased energy loss called "gap loss". As a result, greater equalization and improved detection techniques are required to compensate for a lower signal to noise ratio in the signal recovered from the record media. Accordingly, when reading signals, long gaps are avoided for the above reasons. For high frequency recording, a ferrite pole is usually selected for heads for reducing eddy current losses. Unfortunately, the ferrite pole piece has a low saturation induction which limits the capability of the head to write deep into the magnetic medium.